title: Comparative analysis of crystal-field parameters for rare-earth ions at monoclinic sites in AB(WO4)2 crystals: I. Tm3+ in KGd(WO4)2 and KLu(WO4)2, and Ho3+ and Er3+ ions in KGd(WO4)2.
abstract: The crystal-field (CF) parameters determined by various authors for rare-earth ions at monoclinic sites in AB(WO(4))(2) crystals are reanalyzed using a methodology incorporating several approaches, namely standardization, multiple-correlated fitting technique and closeness of CFP sets. In Part I recent spectroscopic data for Tm(3+) ions in KGd(WO(4))(2) (KGdW) and KLu(WO(4))(2) (KLuW), and Ho(3+) and Er(3+) ions in KGdW, which were interpreted using the free-ion (FI) and CF parameter (CFP) sets, are thoroughly revisited. Our reanalysis enables clarification of several doubtful aspects involved in the previous studies. The initial CFPs for fitting, calculated using the simple overlap model (SOM), differ markedly from the fitted CFPs for Tm(3+) ions in KGdW and KLuW. An inspection of the pertinent CFP sets reveals deeper intrinsic differences between the model and fitted CFPs. The model CFPs and the fitted CFPs for RE(3+) ions in both KGdW and KLuW crystals turn out to be non-standard. Importantly, the model and fitted CFP sets for Tm-KLuW belong to disparate regions of the CFP space and thus are intrinsically incompatible, i.e. such sets should not be directly compared. Thus the CFP sets reported in the literature require reconsideration in view of the intrinsic properties of monoclinic CF Hamiltonians previously not taken into account. Standardization of the originally non-standard CFP sets is carried out to ensure direct comparability of the CFP sets in question with other literature data. The correlated alternative CFP sets are calculated for each original set to facilitate future applications of the multiple correlated fitting technique, which enables improving overall reliability of the fitted CFPs. The closeness of the standardized CFP sets is assessed in a quantitative way. Our considerations indicate also the importance of proper definitions of the axis system used in the CFP model calculations and provide arguments for the nominal meaning of the axis systems assigned to the fitted CFPs. The consistent methodology proposed here may be considered as a general framework for analysis of CF levels and CFP modelling for rare-earth and transition-metal ions at monoclinic symmetry sites in crystals. CFP sets for other rare-earth ions in AB(WO(4))(2) crystals will be reanalyzed in Part II.
authors.affiliation: Institute of Physics, West Pomeranian University of Technology, Szczecin, Poland. crudowicz@zut.edu.pl
authors.forename: Czesław
authors.initials: C
authors.lastname: Rudowicz
authors.forename: Paweł
authors.initials: P
authors.lastname: Gnutek